In a large saucepan melt butter with the olive oil over moderate heat. Add the shallots, sliced mushrooms, thyme leaves, salt and pepper. Cook until the liquid is rendered from the mushrooms.

Remove a 1/2 cup of the cooked mushrooms and reserve.

Stir in the flour and cook for 2 to 3 minutes. Add the chicken broth, bring to a simmer and cook for 15 minutes.

In a blender puree the soup in batches until smooth. For a very smooth soup, pass through a fine sieve. Return soup to saucepan, bring to a simmer and whisk in the milk. Add additional milk to achieve desired consistency. Adjust seasoning with salt, pepper and nutmeg.

Lower heat and whisk in 1/4 cup of the sour cream.

Keep soup warm but do not simmer or boil. Serve soup garnished with reserved cooked mushrooms and a dollop of the remaining sour cream.
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